The invention relates to a method for operating a vehicle having an internal combustion engine with a generator, as is known from DE 10 2008 008 238 A1.
When hybrid vehicles are operated using the combustion engine, the electric drive is operated in the reverse direction as a generator to generate electric current for charging a battery; the battery is then used to power the electric drive, when the internal combustion engine is switched off.
Electrical current is usually produced by the internal combustion engine which generates additional torque in addition to the actual torque used to drive the motor vehicle. In other words, a partial torque of the total torque generated on a shaft during operation of the internal combustion engine is used to generate electric current from the generator. In principle, it is desirable to consume as little fuel as possible for the generation of the electric current. Therefore, preferably a first mode exists in which the partial torque is set so that the efficiency is adjusted to an optimum (maximum) value. The efficiency is defined as the work provided by the electric current per quantity of fuel. It is known from DE 10 2008 008 238 A1, that more than one charging or discharging function may exist, which may be selected by a control device. An additional operating range exists besides the operating range with the most favorable efficiency. The input variable, which defines this additional operating range, is an actual difference between a target charge of the battery of the hybrid vehicle and the actual charge. The charge is expressed as a percentage in relation to a maximum capacity.
Therefore, a second mode exists in addition to the first mode in which the efficiency is set to an additional value (which is different from the maximum value). To this date, the absolute power and not the efficiency was used as guiding principle: namely, the second mode is preferably implemented when the battery needs to be charged quickly. In this case, the absolute value of the work performed is important. Less attention is paid to the efficiency, i.e. the ratio of the work to the required fuel quantity.
However, a relatively small quantity of fuel should be consumed even when the battery is to be charged faster. When the partial torque is specified only for producing a large output power, as was the case until now, the desired function of the hybrid vehicle is undermined due to the increased fuel consumption.